Traditionally, many packaging materials are prepared from thermoplastic polymers, such as polystyrene, polyethylene, and polypropylene. These polymers are generally very stable and can remain in the environment for a long time. Recently, however, there has been a trend to develop articles and products that are considered environmentally friendly and sustainable. As part of this trend, there has been a desire to produce ecologically friendly and biodegradable packaging products.
One promising polymer that may meet this desire is polylactic acid (PLA). Polylactic acid, also known as polylactide, is a biodegradable, thermoplastic, aliphatic polyester derived from renewable resources, such as sugar, starch, or cellulose. PLA is promising because it is manufactured from natural substances, such as corn, and therefore may provide a sustainable alternative to petrochemical-derived products.
However, PLA has several disadvantages that have limited its use in packaging and other applications. In particular, thermofoamed articles comprising PLA resins may be dimensionally stable (i.e., heat resistant) at temperatures up to only about 49° C. This low dimensional stability at elevated temperatures can be particularly problematic during summer months when temperatures to which packaging articles comprising PLA may be exposed to relatively high temperatures during storage or transportation.
Several approaches have been developed to address these problems. One approach includes the use of a crosslinking agent, such as peroxide, in the foam. For example, crosslinking the foam matrix can help to increase the extensional viscosity (melt strength) of the polymer and minimize cell wall collapse. Other approaches include blending PLA with one or more additional polymer resins, such as styrene or a polyolefin. However, the addition of crosslinking or other non-biodegradable polymers in the foam reduces the ability of the foam to decompose. Further, inducing crosslinking in the foam generally requires an additional step which can add to the complexity and cost of producing the foam.
In addition, common traditional methods of preparing thermoformed PLA articles require a two-step molding process in which a heated PLA foam sheet is first subjected to a hot mold step followed by a cold mold step. In the hot mold step, the heated PLA foam sheet is introduced into a hot mold that is heated to at least 100° C. While in the hot mold, the foam sheet is molded to a desired shape and configuration. In addition to shaping the article, the hot mold step also induces crystallization into the PLA foam. In a second step, the thus molded foam is transferred to a cold mold that “freezes” the foam into the desired shape. The temperature of the cold mold is typically less than 30° C. Generally, to improve the dimensional stability of the thermoformed article, the PLA foam is exposed to the hot mold for an amount of time to induce a sufficient amount of crystallization in the PLA foam so as to improve heat resistance of the molded article. For example, in some cases it may be necessary to expose the PLA foam to the hot mold for at least 15 seconds. As a result, longer production times are generally needed to prepare heat resistant PLA foamed articles.
Accordingly, a need still exists for improved methods of preparing PLA foams having improved properties.